Understanding Recent Eastern Horn of Africa Rainfall Variability and Change

Observations and sea surface temperature (SST)-forced ECHAM5 simulations are examined to study the seasonal cycle of eastern Africa rainfall and its SST sensitivity during 1979-2012, focusing on interannual variability and trends. The eastern Horn is drier than the rest of equatorial Africa, with two distinct wet seasons, and whereas the October-December wet season has become wetter, the March-May season has become drier. The climatological rainfall in simulations driven by observed SSTs captures this bimodal regime. The simulated trends also qualitatively reproduce the opposite-sign changes in the two rainy seasons, suggesting that SST forcing has played an important role in the observed changes. The consistency between the sign of 1979-2012 trends and interannual SST-precipitation correlations is exploited to identify the most likely locations of SST forcing of precipitation trends in the model, and conceivably also in nature. Results indicate that the observed March-May drying since 1979 is due to sensitivity to an increased zonal gradient in SST between Indonesia and the central Pacific. In contrast, the October-December precipitation increase is mostly due to western Indian Ocean warming. The recent upward trend in the October-December wet season is rather weak, however, and its statistical significance is compromised by strong year-to-year fluctuations. October-December eastern Horn rain variability is strongly associated with El Nino-Southern Oscillation and Indian Ocean dipole phenomena on interannual scales, in both model and observations. The interannual October-December correlation between the ensemble-average and observed Horn rainfall 0.87. By comparison, interannual March-May Horn precipitation is only weakly constrained by SST anomalies.